1. Field of the Invention
The present invention relates to an optical semiconductor device where an integrated circuit, a photo detector and a micromirror are mounted on the same substrate, and to a manufacturing method for the same.
2. Description of the Related Art
An optical semiconductor device is a device where a photo detector and an optical element, such as a micromirror, are formed on a substrate, such as a semiconductor substrate, together with an integrated circuit made of transistors, and is used for an optical pickup part for a DVD (digital versatile disc) or a CD (compact disc). It is desired to increase the sensitivity and speed of photo detectors, in order to deal with an increase in the speed of DVDs and CDs. In the case where such an optical semiconductor device is formed, an insulating film is formed on the surf ace of the substrate, in order to protect the surfaces of the elements, or in order to protect the wires, in the same manner as in a conventional semiconductor device. In the case where a similar insulating film is formed on photo detectors formed on the same substrate, however, a problem arises, where the photoelectric conversion efficiency decreases or disperses, due to optical interference. Therefore, the formation of an antireflection film on photo detectors is carried out, in order to enhance the photoelectric conversion efficiency. An antireflection film is formed of a silicon oxide film and a silicon nitride film on photo detectors, and then, wires for transistors are formed. Furthermore, the entire surface is covered with a thick plasma silicon nitride film, which is a protective film, and portions of the plasma silicon nitride film on the photo detectors are removed. According to this method, however, the insulating film, which is, for example, an interlayer insulating film, formed on the transistors is not formed on the antireflection film in a manner where only the antireflection film exists on the photo detectors. Therefore, the antireflection film itself is sometimes etched through the etching at the time of selective removal of the plasma silicon nitride film, and it is difficult to maintain the film thickness of the antireflection film constant. In the case where the film thickness of the antireflection film disperses as described above, the reflectance increases, and the photoelectric conversion efficiency decreases. That is to say, the manufacturing method thereof has a problem where it is difficult to control the film thickness of the antireflection film.
Therefore, a method for precisely controlling the film thickness of the antireflection film by forming an etching stopping film, such as an aluminum film, that cannot be etched with a CF4 based etchant on the antireflection film has been used. Meanwhile, the silicon substrate is etched with a potassium hydroxide solution when a micromirror is formed. In this case, the etching stopping film, which is an aluminum film, is disadvantageous. That is to say, aluminum films are easily corroded by alkaline or acid. Accordingly, the etching stopping film is corroded by the potassium hydroxide solution at the time of the formation of a micromirror.
Furthermore, the material of the etching stopping film may be changed to one which is not etched by alkaline or acid, in order to prevent corrosion of the etching stopping film. In this case, however, the selective ratio of the base to the antireflection film becomes smaller, making it impossible to selectively remove only the etching stopping film. Therefore, the antireflection film itself is also etched, and it becomes difficult to maintain the film thickness thereof constant.